There's no magical formula, I'm afraid, just a different way of thinking than you're probably used to. Many motor manufacturers provide these thrust ratings because they can't come up with any other simple way to rate their motors. Unfortunately, they're as meaningless as the HP ratings on glow engines...

First off, it's okay to plagarize. Find a plane that's similar to what you want to power, and copy its power system. That's the easy way out. For example, this ultrafly Extra 300 sounds like it's similar in size and weight to the Horizon Hobby's Mini Funtana. There are plenty of options out there for good 3D power systems on the Mini Funtana.

To really understand what's going on, start by remembering that it's all about the power. Forget about thrust, because if you give the plane enough power, and a reasonably-sized propeller, you'll have plenty of thrust without worrying about it.

Electric power is measured in Watts. Powering an electric plane requires so many Watts per pound to achieve a certain level of performance. I hesitate to call it a "power to weight ratio" but it kinda sorta is. For a 3D plane, 150 Watts per pound will provide all the thrust you'll need for most any maneuver you can think of, regardless of the size of the airplane. A 22oz airplane is 1.375lbs, so you need around 200 Watts of power.

From here with AXi motors, it's pretty easy. Measure the prop clearance you've got for takeoff and landing, and plan on the biggest possible prop. Next, go to www.modelmotors.cz and look through the charts on the available motors for one that takes at least 200 Watts (input power) and spins a prop that's close in diameter to what you figured for ground clearance. Read from the chart to get your motor, prop, Amps, and battery.

Check this out: http://www.modelmotors.cz/index.php?id=en&nc=produkty_vypis&kategorie=m_neod ym_ac&id_rady=axi_28&id_produktu=axi_2808_24&nazev _rady=Series%20AXI%2028&hmotnost_rady=(78%20-%20161%20g) This is the AXi 2808/24. With a 10-cell NiCd battery and a 9.5x5 prop, it'll draw around 19 Amps for an input power of 209 Watts. Except for converting NiCd to LiPoly, it sounds like the perfect power system to me.

Converting the NiCd pack to LiPoly, we know we need a similar voltage to the 10-cell NiCd, and the capacity to handle at least 19 Amps. A 3S LiPoly is VERY close in voltage to a 10-cell NiCd pack (11.1V vs 12.0V nominal, under load is much closer). Unfortunately, your 1200 and 1320 packs won't handle the 19 Amp load, so you'll either need to put two 3S 1320 packs or 3S 1200 packs (no mixing sizes) in parallel if you've got 'em, or you need a 3S LiPoly of around 2000mAh in capacity.

Don't get hung up on the prop size, as I know 9.5x5 is a tough find in the USA. You have some wiggle room in either direction, just like on a glow engine. You could go down to a 9x6, or up to a 10x3.8, and the power consumption will be about the same.